This invention relates to electronics for automotive application and more specifically relates to a novel protection circuit against load dump test conditions.
Automotive electronic systems are required to survive an over voltage condition, commonly known as xe2x80x9cload dumpxe2x80x9d in which inductive components in an automotive circuit produce a voltage spike when the circuit is opened while carrying current. In this test, voltages of over 50 volts can be applied to the automotive electronics. In order to survive this test, power electronics devices, in the circuit under test, such as power MOSFETs and IGBTs, are rated to have breakdown voltages of about 55 volts or, if a zener diode clamp is used, at 38 volts, the devices can be rated at about 40 volts. The on resistance of a MOSFET increases as its reverse voltage rating increases. Thus, devices needed to withstand the load dump condition will have a higher on resistance (RDSON) than lower rated voltage devices (rated at the ordinary operating voltage of the system) resulting in a higher power loss.
It would be desirable to configure a protection circuit which would permit lower voltage power electronics devices to adequately pass the load dump test.
The circuit of the invention is a very small circuit which can reside on a PCB in every automotive electronic module which is required to pass the load dump test. In accordance with the invention, the internal resistance of the test set up is used to divide the voltage seen by the electronics system to voltages as low as 28 volts, thus permitting the use of lower RDSON MOSFETs rated at about 30 volts instead of the higher voltage (40 volts with a zener clamp and 55 volts without the clamp) and higher RDSON.
The resulting circuit has a lower cost than prior art solutions which have included use of zener diode to clamp the bus voltage to 38 volts because:
1. Prior art solutions require the use of 40V (when a zener diode clamp is used) or 55V (if no zener diode clamp is used) rated devices which have a significantly higher Rds(on) than 30V rated devices. That means that for the same Rds(on), a 30V device will have a significantly lower cost than a 40V or 55V part, allowing the system to be lower cost.
2. If sized properly, the bill of material cost of the invention (using 30V devices) is lower than that using the zener diode and a 40V device.
3. Zener diodes can not be safely used to clamp the voltage at 30V because the wide variations of the reverse characteristics of the zener diode and its xe2x80x98soft slopexe2x80x99 would cause the zener to dissipate considerable power in double battery condition (24V).